In the prior art process, receptacles to be used in electromagnetic cookers, such as inner pots of rice cookers, are produced from a composite plate base material composed of a heat-generating magnetic metal plate such as iron, stainless steel or the like and a heat-conducting aluminum or aluminum alloy plate, by subjecting the base material to a press mold working such as deep drawing in such a way that the aluminum or aluminum alloy plate is arranged inward. The inner surface of the pot is generally coated with a fluorine resin to prevent sticking of cooked rice.
In general, such a composite-plate base material is produced by subjecting a magnetic metal plate and an aluminum or aluminum alloy plate to rolling thereby making these plates into a clad material, namely a composite material (as described, e.g., in JP-B-54-3468 and JP-B-54-9985). (The term "JP-B" as used herein means an "examined Japanese patent publication")
Rolling is useful for the production of clad materials in a large quantity. However, being produced by rolling, thickness of the resulting plates varies greatly due to compressed cladding of the aluminum or aluminum alloy plate, thus causing serious problems from the processing point of view such as generation of cracking and crinkling during press molding of the clad plates. In addition to these problems, such clad plates are apt to cause adhesion of foreign materials and generation of flaws, thus requiring a surface polishing step, and clad strength of the plates becomes partially weak in some cases thus causing separation during the press working.
Recently, the inventors of the present invention have proposed a process for the cladding of a magnetic metal plate and an aluminum or aluminum alloy plate by an isotropic hot pressing (as described in JP-A-5-116244). (The term "JP-A" used herein means an unexamined published Japanese patent application.) In the isotropic heat pressing process, high pressure heating is carried out in an atmosphere of an ultra high pressure inert gas (mainly Ar gas) using a pressure vessel equipped with a heating system. By this treatment, a synergistic effect of high temperature heating and the isotropic pressure by gas pressure is applied to the plates, thereby applying plastic working and diffusion of composing elements to the materials constituting the plates.
Most characteristic advantages of the isotropic heat pressing are that clad plate materials can be obtained without using the rolling process which is generally employed in the field of such type of composite plate materials and that a magnetic metal plate such as iron, stainless steel or the like and an aluminum or aluminum alloy plate can be made into a clad plate directly without interposing a diffusion metal layer. This process, however, has disadvantages in that it requires high facility operation cost because of the use of a high pressure gas and it requires disposable metal capsules which cause increase in the production cost due to considerable labor for the vacuum sealing and removal of the capsules.
The present invention contemplates overcoming the aforementioned problems involved in the prior art, as well as disadvantages of the above-described isotropic hot pressing.